Add dev/stg/prod environments with per-env values, configmaps and secrets

- Move env-specific config into environments/<env>/{values,configmap,secret}.yaml
  for both nginx-chart and mysql-chart (dev, stg, prod)
- Make templates env-agnostic: resource names come from values
  (configMap.name, secret.name)
- nginx deployment mounts env ConfigMap and loads env Secret via envFrom
- mysql statefulset reads DB config from ConfigMap, passwords from Secret
- Remove mysql-chart/templates/secret.yaml (secrets are now per-env manifests)
- Add README with layout and deploy commands

nginx-chart/environments/dev/values.yaml

@@ -0,0 +1,16 @@
+# DEV environment overrides for nginx-chart
+namespace: dev
+
+replicaCount: 1
+
+image:
+  tag: "39"
+
+ingress:
+  host: nginx.dev.local
+
+configMap:
+  name: nginx-config-dev
+
+secret:
+  name: nginx-secret-dev